fix(foundation.slider.js): consider 0 value for `pageX/Y`

* fix `Uncaught TypeError: Cannot read property '0' of undefined` if mouse position X/Y is 0
* add `get_cursor_position()` method

If you start dragging the slider and move the mouse position X/Y until it is `0`, it is counted as falsey which means `||` in [line 49](https://github.com/zurb/foundation/blob/master/js/foundation/foundation.slider.js#L49) and in [line 55](https://github.com/zurb/foundation/blob/master/js/foundation/foundation.slider.js#L55) keeps going until it gets to `e.originalEvent.touches[0].clientX/Y` where it raises an error if it is no touch device. I changed it to explicitly checking that the values are `undefined`.

![oq6fzbeh](https://cloud.githubusercontent.com/assets/464300/5014284/cf3519fc-6a94-11e4-8b9a-95149ad64ac5.png)